1. Field of the Invention
The present invention generally relates to the handling of transport containers and, more particuarly, is concerned with an apparatus and method for controlling the operative functions of a crane apparatus through the use of an improved hydraulic control system.
2. Description of the Prior Art
In recent years, the use of large transport containers of several standardized forms has gained widespread use in industry. These containers permit the efficient transfer of cargo from ships to transporting vehicles, between different transporting vehicles, and to and from storage facilities. Because of the large size of cargo containers, it has been necessary to develop equipment having the capability of effectively handling the heavy loads required for their lifting and transport. One common apparatus for lifting and transporting containers from place to place is in the form of large, self-powered gantry cranes having several separate powered functions. The crane must deliver power to drive wheels, steering mechanisms and brakes. The equipment must also be capable of moving a bridge for positioning over the loads to be carried and of operating a hoist mechanism.
Conventional cranes require either DC power generation with its inherent complexities, such as brushes, commutators, etc., or require the use of resistor banks and various other high power electric devices in order to attain speed control. The devices themselves are complicated, cumbersome and expensive, because operative control must be imposed upon the full output power of the device. Other electric systems seeking to avoid the complexities of DC generation utilize AC generator sets. The AC current is then converted into a variable DC signal with the use of various types of static inverter systems. These devices, even though they have been improved over the last number of years through the use of solid state technology, are still very cumbersome, space consuming, and expensive techniques. Again, this is primarily because the control is actually controlling the full power flow.
The reliance on expensive electrical systems to govern the various power components of a crane increases its cost of manufacture and maintenance, and particularly requires sophisticated maintenance personnel. Consequently, a need exists for an improved system for controlling hoisting, traversing, driving and steering mechanisms of a gantry crane with improved operative characteristics and a simplified design for economy of manufacture, reliability, and ease of maintenance.